Articles: a/an, the, no article

Use a / an with singular countable nouns.
The first time you mention a thing/person. I saw an old man with a dog.
When you say what something is. It's a nice day.
When you say that somebody knows. She's a lawyer.
In exclamations with What...! What a wonderful day!
In expressions like... three times a week

Use the
When we talk about something we've already mentioned. I saw an old man with a dog, and the dog was barking.
When there's only one of something. The moon goes round the earth.
When it's clear what you're referring to. He opened the door.
With places in a town. I'm going to the theater.
With superlatives. It's the best restaurant in town.

Don't use the
When you are speaking in general (with plural and uncountable nouns). Women talk more than men.
With some nouns after at / to / from. She's not at home today.
Before meals, days and months. I never have breakfast on Sundays.
Before next / last + days, week, etc. See you next Friday.